Industrial thermal processing of foods has, until recently, been a batch process, using cans or other sealed containers. Recently, continuous aseptic processing techniques have been employed for continuously sterilizing foods and beverages while flowing; the processed food is then packaged in sterilized containers. In the aseptic process, the food is sterilized at a relatively high temperature of between 265.degree. F. and 300.degree. F. for a relatively short time.
In thermally processing food, it is critical that the food or the process be monitored to ensure sterility. In the batch processing techniques, the achievement of sterility was commonly indicated using color-change indicators engineered to create a color change at a certain given temperature, or after a certain time-temperature exposure. See, for example, U.S. Pat. Nos. 4,514,361, 3,862,824, and 3,000,706. These color change indicators were acceptable for batch sterilization because the entire food containers were subjected to sterilization after sealing. Accordingly, the color change indicators could be applied directly to the containers, or could be placed in the batch processor along with the containers being processed.
Another approach at estimating sterility has been to use mathematical modeling. However, modeling is subject to uncertainty as a result of the assumptions which have to be made in aseptic processing due to the difficulty of measuring the temperature within the flowing stream of liquid food, or at the center of a moving food particulate. As a result, modeling techniques require conservative assumptions of physical parameters, often leading to severe overprocessing of food.
There have also been attempts at using microbiological techniques to determine sterility. For example, particulate foods have been inoculated with thermophilic bacteria as a bioindicator of aseptic processing. The equivalent lethality delivered to C. Botulinum, a standard measure of sterility, is estimated based on the decrease in bacteria population. However, this procedure requires the careful inoculation of the particulates before testing, and bacteria counting after testing, making the entire procedure tedious and subject to experimental uncertainties Moreover, the extent of food overprocessing is difficult or impossible to assess, because once the entire microbial population is destroyed, no further changes can be detected.
There have also been a number of attempts to estimate the sterility of aseptically processed foods by measuring the thermal destruction of compounds added to food before processing. However, these techniques suffer from a number of drawbacks which make them poor indicators of food sterility. For one, typical chemical reaction in foods is either too fast or too slow to be a useful indicator of sterility. For example, destruction of heat-resistant enzymes such as peroxidase is complete in about two minutes at 212.degree. F.; such a fast reaction cannot be used to indicate the degree of thermal processing at high-temperature aseptic conditions. On the other hand, destruction of nutrients such as ascorbic acid and thiamin is much slower than the thermal death of bacteria spores at the high aseptic processing temperatures. For example, the microbial lethality rate at aseptic processing temperatures is about ten times that of the rate at retort temperature, whereas the destruction of thiamin is only about two times faster at the higher temperature. Accordingly, the relatively short time-high temperature aseptic processing conditions will lead to little change in the thiamin concentration, making accurate testing extremely difficult.